Why are autonomous cars categorized as AI?
There is a neat definition of artificial intelligence, which circumvents the problem of defining "intelligence" and which I would ascribe to McCarthy, the founder of the field, although I can only find it now in this book by H. Simon:

"… having to do with finding ways to do intelligent tasks, to do tasks which, if they were done by human beings, would call for our human intelligence."

So, at its core we call the automation of every task AI, that can only be done by the human mind. At the time people thought that a computer able to play chess would also be intelligent in other ways. When this turned out to be false, the term AI was split into "narrow or weak AI", i.e. a program able to do one task of the human mind, and "general or strong AI", a program that can do all the tasks of the human mind. 

Self-driving cars are narrow AI. 

Note, that all these definitions don't specify whether these programs copy the way the human mind works or whether they come to the same result via completely different algorithms.